In standard methods of obtaining metallic lead from concentrates, the standard procedure has been to treat the lead sulfide concentrates in a blast furnace. However, the pyrometallurgical procedure possesses many disadvantages and drawbacks. Primary among these disadvantages is that the process will result in some major pollution problems such as the generation of sulfur oxide gas along with substantial fuming. The fuming carries with it possible carcinogenic compounds which will contain lead, cadmium, etc. Therefore, it is necessary to provide improved and safer methods for obtaining metals such as lead in metallic or elemental form by methods which will not contribute to pollution of the air or will be safer to operate. The aforementioned lead smelting techniques will consist of roast sintering the lead sulfide concentrate whereby a major portion of the sulfur will be removed followed by melting in a blast furnace to obtain the metallic lead.
In an effort to alleviate the pollution problem it is necessary to develop new processes for obtaining lead which will be competitive as an alternative to the conventional smelting practices. Prior work in the hydrometallurgical field resulted in developing a non-aqueous processing route whereby lead sulfide concentrates are chlorinated at temperatures above 300.degree. C. to produce lead chloride and volatilized sulfur. However, chlorination at these elevated temperatures will promote the formation of volatile chlorides of contaminating elements such as iron, magnesium, aluminum, silicon, zinc as well as elemental sulfur which may be present in the lead sulfide concentrate. Other hydrometallurgical processes which have been developed include the use of ferric sulfate as a leach agent. In this method, the lead sulfide is sulfated to form lead sulfate. This step is then followed by carbonation of the lead sulfate to form lead carbonate and thereafter the lead carbonate is subjected to dissolution in hydrofluosilicic acid for electrolysis to metallic lead. Yet another hydrometallurgical method which is developed for the recovery of lead is based on the use of an acidic ferric chloride medium. This method involves a leaching step whereby the lead sulfide is converted to lead chloride and thereafter subjected to steps of solubilizing, crystallization and electrolysis.
The prior art which discloses other processes for converting a lead sulfide to elemental or metallic lead is also exemplified by U.S. Pat. No. 1,491,653. This reference describes the use of chlorides of sulfur and in particular sulfur monochloride to selectively chlorinate lead sulfide in a complex lead-zinc sulfide ore at temperatures ranging from 50.degree. to about 150.degree. C. However, this method is also an aqueous type of operation in that the solids are reacted in a solution of sulfur monochloride to form a slurry. A disadvantage which is present when utilizing such a system is that certain metal sulfides are solubilized due to the dissolution of sulfur into the slurry, this dissolution being due to the wide range of compositions of sulfur chlorides. Such an action can lead to the dissolution of some metal sulfides thereby rendering the process more complex in nature. In addition to this type of operation, another metal recovery system utilizes a dry chlorination of complex sulfides in a two-stage process. The first stage consists in a countercurrent chlorination of the ore in a tube mill with chlorine gas, the temperatures of this process usually being in a range of from about 100.degree. to about 150.degree. C. to insure chlorination of from about 60% to about 70% of the metals. The important step in this stage is the chlorination of iron which serves as a source of chlorine in the second step. The second step of this two-stage process consists in a chloridizing roast wherein the final chlorination is accomplished to convert all metals present in the ore to chlorides. Much of this reaction is done by the release of chlorine by the oxidation of the initially formed ferric chloride to ferric oxide and chlorine. Following this, the metal chlorides are then leached in water and brines in order to solubilize the metals. However, the chloridizing roast to produce ferric chloride will also produce sulfur and sulfur chlorides as well as oxides, the roast temperatures which are necessary to accomplish this being above 138.degree. C. and probably about 150.degree. C.
In addition to the prior art hereinbefore discussed, another reference, namely, U.S. Pat. No. 3,961,941, discloses an activation step in which an ore containing metallic lead and silver is subjected to a roasting step so that the silver bearing mineral may be more easily treated in a subsequent oxidative leach employing an aqueous solution of ferric chloride as the leaching agent, followed by a second leaching with an aqueous solution of sodium chloride to dissolve the salts.
As will hereinafter be shown in greater detail, it has now been discovered that the yields of metallic lead which are obtained from lead bearing sources may be increased when utilizing a prehalogenation activation treatment prior to further steps which include halogenating a lead sulfide source at a relatively low temperature in a dry atmosphere to selectively halogenate the lead.
This invention relates to a hydrometallurgical process for the recovery of metallic lead. More specifically the invention is concerned with an improvement in the process for obtaining metallic lead from lead sulfide concentrate whereby unwanted side reactions such as the halogenation of other metals present in the concentrate are minimized. An attendant advantage of effecting the process in a manner hereinafter set forth in greater detail is that the lead halide which is produced during the reaction is recovered in a purer form than has heretofore been obtainable with the resultant recovery of metallic lead being such that said lead is in a relatively purer state.
It is therefore an object of this invention to provide a process for the production of metallic lead.
A further object of this invention is to provide a hydrometallurgical process for the production of metallic lead from lead sulfide concentrates whereby the lead is recovered in a relatively purer state.
In one aspect an embodiment of this invention resides in a process for the production of metallic lead from a lead bearing source which comprises the steps of: (a) halogenating said lead bearing source; (b) leaching the halogenated source with brine; (c) filtering the resultant solution to separate elemental sulfur, residue and soluble lead halide; (d) crystallizing said lead halide; and (e) recovering metallic lead by electrolysis, the improvement which comprises subjecting said lead bearing source to an activation heat treatment prior to the halogenation step.
A specific embodiment of this invention resides in a process for the production of metallic lead from a lead bearing source in which said lead bearing source is activated by heating said source at a temperature in the range of from about 300.degree. to about 600.degree. C. in an inert or reducing atmosphere, halogenating the activated lead bearing source by treatment with chlorine gas at a temperature in the range of from about 80.degree. to about 120.degree. C., leaching the treated mixture at a temperature of from about 80.degree. to about 120.degree. C. with a sodium chloride solution, filtering the resultant solution at a temperature in the range of from about 80.degree. to about 120.degree. C. to separate elemental sulfur, residue and soluble lead halide, crystallizing said halide, and recovering metallic lead by an electrolysis process utilizing a sodium chloride-lead chloride mixture as the molten salt in which said electrolysis is effected.